PVC compounds are susceptible to discoloration absent proper stabilization. Also, while good stabilization features may be obtained by proper compounding, there still may be inherent deficiencies in the clarity, color and heat stability of the resin polymer which present limitations for commercial applications. With regard to dispersion PVC, these resins may be more specifically referred to as emulsion type or micro-suspension type dispersion PVC. These terms correspond to different methods of polymerization. Dispersion PVC is typically used in plastisol and organosol formulations, and there are limitations in inherent initial color after fusion, and color stability evidenced by premature yellowing of films either for unsupported films or film coatings on articles. Initial color is particularly noticeable when a clear film is applied to a light colored, opaque substrate like white flooring sheets. One important aspect not dependent on the resistance to discoloration after heat aging, is the initial color obtained only after sufficient heat for fusion. This color is also referred to the water-whiteness initially obtained. In addition to yellowing tendencies, in many applications, an article containing a plastisol coating can be susceptible to blushing on contact with water. Compounding formulation may limit to some extent the blushing of film or coatings, however it has been observed that the dispersion resin preparation method plays the most important part in determining the initial color and the blushing characteristics of the fused compound.
U.S. Pat. No. 4,076,920 discloses a method of preparing dispersion PVC which yields dispersion resins having improved heat stability, color and resistance to blushing on contact with water. These resins are made by polymerization using a colloidal dispersant system free of alkali metal soaps, and including the use of ammonium salt of a fatty acid and a long chain alcohol, with polymerization in basic medium which reduces polymer coagulum. The resins, when formulated into plastisols, fused into finished films, and subjected to elevated humidity showed improved blush resistance characterized by rapid recovery of film clarity. There are however limitations in the initial color of this type of resin when formulated as clear layers over white or light colored substrates.
Studies with other dispersion resins of the current state of the art, as clear plastisol films, still evidence some haze and coloration limitations. It would be desirable therefore to improve the initial color while also, preferably not sacrificing blush resistance in a dispersion PVC resin adaptable to plastisol applications. It would also be desirable to provide an effective method for improvement in clarity or haze of the articles derived from dispersion resins.
A method has been devised for manufacturing a dispersion PVC polymer which exhibits improved retention of water-white color, good resistance to blushing and improved clarity.